This collaborative R01 investigates the safety and efficacy of the glutamate-modulating and neuroprotective agent riluzole in a randomized double-blind placebo-controlled clinical trial (RCT) in patients with treatment- resistant depression (TRD). The recent findings of the STAR*D study highlight the fact that our current armamentarium of antidepressant medications, developed out of the monoamine hypothesis, has serious limitations. There is now emerging evidence that amino acid neurotransmitter (AANt) systems may also contribute to the pathophysiology of major depressive disorder (MDD), and that drugs targeting these systems may have potent antidepressant properties. Riluzole is currently approved by the US FDA for the treatment of amyotrophic lateral sclerosis (ALS). It has been shown to have a variety of neurobiological effects on glutamatergic function associated with the drug's neuroprotective and plasticity-enhancing properties. There have now been two recent open-label studies showing riluzole to be effective in TRD as well as several reports demonstrating riluzole's efficacy in bipolar depression, generalized anxiety disorder, and obsessive-compulsive disorder. However, no RCT has been performed in TRD. This randomized, double-blind, placebo-controlled trial, using a sequential parallel comparison design, evaluates the efficacy and safety of riluzole augmentation of the selective serotonin reuptake inhibitor citalopram in outpatients ages 18-65 with moderate TRD. Significance to Public Health: In this unique 5-year proposal, investigators from four institutions [Yale, MSSM, MGH and NIMH (MAP)] will conduct a RCT to examine the efficacy, safety and tolerability of adjunctive riluzole treatment in TRD. Demonstration of riluzole's benefit would represent a major advance for patients with difficult-to-treat depression, and may help elucidate our understanding of the pathophysiology of MDD and the mechanism of antidepressant action. Most immediately, since riluzole is already a FDA-approved medication that has been safely prescribed to thousands of patients with ALS, positive findings from this study could rapidly disseminate into clinical practice and would encourage further investigation of this strategy. Demonstrating riluzole's efficacy in TRD would open the door to the discovery of future medications targeting the glutamatergic system that could be used to fight this common and devastating disorder.